It is now common to provide workers with tool reaction balancing mechanisms for holding tools which generate a high degree of torque or vibration and which may also have large mass and weight, such as nut-runners and certain types of welding equipment, for example. Known tool reaction balancing mechanisms typically consist of an arm having a hinge at the middle thereof, tool holding means at an end of the arm, and a pressurized-fluid, reaction and balancing system operatively connected to the tool holding means.
More particularly, the arm of a known tool reaction balancing mechanism generally has a first arm section and a second arm section. The first arm section is pivotally mounted at one end to a support member such as to swing arcuately therefrom along a first plane, and is hingedly connected at its other end to one end of the second arm section. The hinged connection between the arm section is adapted to permit the second arm section to move arcuately in relation to the first section along a second plane substantially at right angle to the plane of first arm section movement. The other end of the second arm section has the means for holding a tool.
A principal disadvantage to tool reaction balancing mechanism of the type described is that a tool mounted in the holding means at the free end of the second section of the mechanism arm is not displaceable linearly along either of the two planes at right angle. Yet, especially for assembly line operations, a tool is preferably able to be displaced along a linear path parallel to a linearly moving work station.
If desired, the support member to which the arm of a tool reaction balancing mechanism is pivotally mounted can be stationary and fixed, such as a post. However, in factory applications the arm of the mechanism is frequently suspended from overhead tracks or rails by a travelling bridge.
Prior art travelling bridges generally consist of a rigid cross beam, a depending load support beam, a pair of side beams and at least two trolley members fastened to each side beam. The side beams are fixedly secured to ends of the cross beam at right angle, in an H-shaped frame configuration, with the load support beam protruding downwardly from the cross beam proximate the middle thereof. The trolley members are each adapted to be rollably engageable with a track or rail.
These bridge structures, having trolley members fastened to the side beams of rigid H-shaped frames, are expensive to manufacture and are not satisfactory to use when the spaced-apart rails on which the trolley members ride are not perfectly parallel, since variances in the distance separating the rails along their respective lengths frequently can cause the bridge to become stuck. Prior attempts to solve this problem, by deliberating introducing play into the trolley members, for example, have produced unacceptable side effects, such as bridges which swing from overhead rails to an unsafe degree.